In the battery compartment of my 00899 Rain Gauge there is a slider switch marked "A B C" . What is it for?

The ABC switch selects the channel that your device (rain gauge in this case) uses to send data to your display console. The device switch setting much match the display switch setting for the display to recognize the device.

As Robert has mentioned, this is just an ID switch that allows the display and outdoor collector to connect. You will just want to make sure that they are on the same setting to ensure that they are able to communicate.